ChrisT
Club Retro Rides Member
Posts: 1,637
Club RR Member Number: 225
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Apr 30, 2011 13:22:20 GMT
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So, I'm officially confused as to what's up with my engine (C20XE on twin webers) so I'll put it up here to see if anyone can come up with any ideas. A bit of background, rebuilt the engine recently - honed the bores, replaced one set of rings and new big end bearings. Started up no problems, took it to the Limes meet last month (about 60miles round trip) still keeping the revs low running it in and everything was fine. Used it on a few short trips from garage to workshop and one morning it sounds like it's running on 3 cylinders, it idles fine and carbs are fairly well balanced just it rocks about and under load it sounds rough with no power. Today I've gone over it removing each plug lead in turn, 1 and 2 seem to make little difference, 3 and 4 it almost stalls. Holding the plug leads near the plug I can hear a good spark so took this to mean a dodgy plug. Took the plugs out to find no.2 wet, others fine, assumed this meant it wasn't firing so swapped them all around (1 and 2 in 3 and 4) did the test again and got the same results  Mixture screws on front carbs don't seem to be doing a great deal either which must be related. And check out this video I did..... Can someone explain that? Any suggestions gladly received |
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Apr 30, 2011 13:32:18 GMT
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Can you do a compression test?
Does the XE use stretch bolts or not? If not have you re-torqued the head?
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bmw156
Part of things
Posts: 796
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Apr 30, 2011 14:02:41 GMT
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compression test i would say also. compression should be around 12bar
really weird that it runs "smooth" but plugs one and two don't affect it. have you done the test with a plug plugged in agianst the exhaust manifold to see if it has good spark.
yes XE's use stretch bolts on the head.
should be done in this order, 25Nm, 90 degrees, 90 degrees, 90 degrees. going in - out order - technical!
sounds really weird, what coil pack is it running?
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ChrisT
Club Retro Rides Member
Posts: 1,637
Club RR Member Number: 225
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Apr 30, 2011 15:31:09 GMT
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Compression test yielded fairly even results across all 4 - 18.5Bar, 20, 20+, 19.5 front to back
It's got ARP head studs which, AFAIK don't need re-torquing.
Can hear the spark bridging between the plug lead and plug when held close so I think spark is good.
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Apr 30, 2011 16:36:50 GMT
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Petrol engines need three things to run: The right amount of fuel, compression, and a spark at the right time.
Since it has decent compression I'm going to assume that the valves, HG, rings and cam timing are all good.
So that leaves spark or fuel. Since you say plug 2 was wet I suspect that it's the spark.
It's possible that when you heard the spark it was going to the head and not the plug - do the leads have a metal screening can at the end? That occurred on a mercedes that I looked at.
Another possibility is that you've mixed up the plug leads for Nos. 1 and 2. If it is running a wasted-spark ignition, it'll have 1 and 4 on one coil and 2 and 3 on the other - it's well worth checking.
I know you say that it idles fine, but that can be deceiving - at idle the combustion is really quite quiet. The only way I diagnosed the misfire on my ex Mondeo was to listen to the tail pipe - the noise was intermittant rather than continuous.
So: check the spark with a removed plug against the head or exhaust manifold, and double-check the plug lead connections.
Once you've made sure that the ignition is ok, then move on to the carbs.
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ChrisT
Club Retro Rides Member
Posts: 1,637
Club RR Member Number: 225
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Apr 30, 2011 18:33:29 GMT
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Good info, thanks...
Will double check the spark wasn't going to ground rather than the plug.
Plug leads are in the right order (numbered) and it's a std coil so no wasted spark.
The idle does sound very lumpy from the exhaust.
Did a quick test with the colortune on 1 and 2, both showed a bit rich on idle but went right with a bit of throttle.
Plugs are iridium tipped, wondered if they might be a bit sensitive to rich mixture and are fouled up, will get some std plugs tomorrow see if that makes any difference
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Tim
Posted a lot
Posts: 3,340
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Apr 30, 2011 18:56:00 GMT
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From my fun over the last few weeks - check ALL the vacuum hoses and then CHECK THEM AGAIN :-)
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ChrisT
Club Retro Rides Member
Posts: 1,637
Club RR Member Number: 225
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Apr 30, 2011 18:58:30 GMT
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Shouldn't take long - there aren't any.
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Apr 30, 2011 20:21:01 GMT
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Pull your plugs clean them with a toothbrush, then test individually using 1 lead. If all 4 are sparking, take one and test to see that you have a spark from that plug from each HT lead. With this sort of thing you need to be methodical and the golden rule only test/change 1 thing at a time!
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Tim
Posted a lot
Posts: 3,340
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Apr 30, 2011 21:29:51 GMT
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Shouldn't take long - there aren't any. Excellent  |
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ChrisT
Club Retro Rides Member
Posts: 1,637
Club RR Member Number: 225
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Checked the timing and everything's were it should be so fitted a set of new plugs and it seems to have done the trick ;D
Really thought it was going to be something more than that.
Cheers for all the advice.
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edit: should have read the last post  ;D |
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Last Edit: May 1, 2011 16:31:56 GMT by lolface
78 Kadett C 2.0 8 valve turbo, holset, rust&other stuff..
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;D